The present disclosure relates to multi-color document processing systems such as printers, copiers, multi-function devices, etc., and to control techniques for operating the same. The disclosures of U.S. Patent Application Publication Nos. 2008/0152369 to DiRubio et al., 2008/0152371 to Burry et al., and 2010/0329702 to Dirubio et al., and the disclosure of U.S. patent application Ser. No. 12/612,121, filed Nov. 4, 2009 and entitled “Dynamic Field Transfer Control in First Transfer” to Lee are hereby incorporated by reference in their entireties. Multi-color toner-based xerographic printing systems typically employ two or more xerographic marking devices to individually transfer toner of a given color to an intermediate transfer structure, such as a drum or belt (referred to as first transfer operations), with the toner being subsequently transferred (in a second transfer operation) from the intermediate medium to a sheet or other final print medium, after which the twice transferred toner is fused to the final print.
Retransfer occurs when toner on the intermediate belt from previous, upstream marking devices is wholly or partially removed (scavenged) due to high transfer fields within the current transfer nip. High fields in the transfer nips in the downstream marking devices can adversely modify the charge state of the toner on the intermediate transfer belt (ITB) through air breakdown mechanisms, further exacerbating retransfer. When this happens, the desired amount of one or more toner colors is not transferred to the final printed sheet, and the retransfer problem worsens as the number of colors increases. Retransfer at a given marking device may be reduced by lowering the transfer field strength at that device, but this may lead to incomplete transfer during image building at that device. In other words, the transfer nip may be transferring toner to the ITB at one region in the cross-process direction (image building), which requires high fields, while simultaneously scavenging toner from the ITB in another region (retransfer). In addition, the quality requirements of multi-color document processing systems are constantly increasing, with customers demanding improved imaging capabilities without adverse effects of retransfer and incomplete transfer.
Current xerographic transfer controls are optimized against many noise factors such as relative humidity and age of the components. However, the controls may not be optimized for image content, which is ultimately important to end users and customers. While transfer is quite robust for image building, retransfer is still a problem since this defect reduces image quality and increases toner-to-waste (increases run cost). Retransfer is also magnified when products have more than four colors.
One proposed solution is U.S. Patent Application Publication No. 2010/0329702 to DiRubio et al., published Dec. 30, 2010, entitled “Multi-Color Printing System and Method for Reducing the Transfer Field Through Closed Loop Controls”, which minimizes retransfer by detecting the amount of toner transferred to the intermediate transfer belt and employing closed loop controls. However, even this solution leaves residual toner and thus is not a complete solution to the retransfer problem.
Another proposed solution is U.S. patent application Ser. No. 12/612,121, filed Nov. 4, 2009, to Lee, entitled “Dynamic Field Transfer Control In First Transfer”, which presents a multi-color document processing system and method to control color retransfer by allowing operators to override nominal electrostatic transfer control settings and set more optimum conditions for a variety of specific and particular print jobs. This, thus, disables marking devices which are not needed for a particular print job and operates devices required for printing at a reduced transfer field levels for the first transfer. This solution reduces transfer, but at the penalty of reducing the speed at which the print device operates. In addition, this approach may cause a reduction in the amount of toner that transfers from the photoreceptor (P/R) to the ITB. A phenomenon called “incomplete transfer.”